One known jam cleat comprises a pair of spaced side walls the inner faces of which converge from top to bottom. The inner faces are furthermore fluted, the axes of the flutes being canted from the vertical in the direction in which tension is applied to a rope which will be secured by the cleat. In this manner when the rope is inserted between the walls and tension applied thereto the flutes tend to force the rope downwardly and hence to be gripped more securely between the converging inner faces of the walls.
Another known jam cleat comprises a wedge member which is mounted for pivotal movement adjacent a single wall. The wedge member has a toothed or serrated eccentric outer face which, as the member pivots, moves closer to the inner face of the wall. The rope to be secured is located between the outer face of the wedge member and the inner face of the wall and the wedge member is pivoted, usually by means of a handle provided for the purpose, until the rope is gripped by friction between the two said faces.
In New Zealand patent #135041 there is shown a jam cleat having a pair of blocks with converging inner faces and a gear mounted therebetween and having teeth which mesh with teeth formed in the inner face of one of the blocks. The gear is held in position by means of a spring mounted on the block with the toothed face and, while being free to float to a limited extent in relation to its joint with the spring, nevertheless is required to remain in mesh with the teeth on the one block for the proper operation of the jam cleat.
Jam cleats of these latter two kinds are not, as far as the applicant is aware, designed for heavy work. They are insufficiently robust for heavy work and furthermore if excessive tension is applied to the rope the wedge member might be pulled so tightly against the rope that it could not be released.